How to check if a query string value is present via JavaScript?

You could also use a regular expression:

/[?&]q=/.test(location.search)

var field = 'q';
var url = window.location.href;
if(url.indexOf('?' + field + '=') != -1)
    return true;
else if(url.indexOf('&' + field + '=') != -1)
    return true;
return false